1. Field of the Invention
The present invention relates to a head position detecting method and apparatus for detecting the position of a head from servo data recorded on a recording medium, and, more particularly, to a head position detecting method and apparatus for detecting the position of a head from the phase of servo data.
2. Description of the Related Art
Magnetic disk apparatuses move a magnetic head in the radial direction of a rotating magnetic disk to position it on a target track, and read or write data from or on the magnetic disk. To improve the memory capacity and make the apparatuses compact, it is inevitable to improve the recording density, particularly, the track density. To increase the access speed, the seek time of magnetic heads is demanded to be around 10 milliseconds.
Accordingly, the magnetic head positioning circuit employs a digital circuit using a fast processor. Since the use of this digital servo circuit makes it sufficient to detect the head position only at the sample timing, the servo head position detector is also demanded to have a performance different from that of an analog servo position detector.
In general, a typical two-phase servo pattern is designed in such a way that as TPI (track density) of a magnetic disk increases the band of the position demodulator increases, thus making the servo pattern susceptible to noise. According to this two-phase servo pattern, the peaks of the waveform recorded on the servo surface of the magnetic disk are detected, and the head position is detected from the detected peaks. While the peaks are successively obtained in this system, noise and a variation in level on the surface of the magnetic disk directly influence the amount of position detection.
As a solution to this problem, a system, which uses a phase type servo pattern and detects the head position from a phase difference, has been proposed in U.S. Pat. Nos. 4,549,232 and 4,642,562 (corresponding Japanese Unexamined Patent Publication No. 10472/1985).
According to the phase type servo pattern, as shown in FIG. 1, servo patterns with different phases are recorded in the circumferential direction of individual cylinders 0, 1, 2, . . . of a magnetic disk. The illustrated phase pattern has a first field EVEN1 and a third field EVEN2, with a second field ODD of the opposite phase provided therebetween. This pattern allows the position of a moving magnetic head to be read at the center of the second field.
The principle of this type phase position detection will be explained with reference to FIGS. 2 through 4. In FIGS. 2 to 4, the phase pattern is designed to form one cycle by four reference clocks. It is therefore possible to detect the head position in four cylinders 0 to 3. The reference phase of the reference clock is indicated by the thick solid lines in the diagrams. As shown in FIG. 2, when the magnetic head is at the position a (cylinder 2), the phase difference between the reference phase and the phase of the read pulse of the servo pattern becomes 1/2 of one cycle.
When the magnetic head is at the position b (cylinder 1), as shown in FIG. 3, the phase difference between the reference phase and the phase of the read pulse of the servo pattern becomes 1/4 of one cycle. Likewise, when the magnetic head is at the position c (cylinder 2), the phase difference between the reference phase and the phase of the read pulse of the servo pattern becomes 3/4 of one cycle. Further, when the magnetic head is at the cylinder 0, the phase difference between the reference phase and the phase of the read pulse of the servo pattern becomes zero or one cycle.
While the above is an example for the fields EVEN1 and EVEN2 of the servo pattern, the phase difference is similarly found for the field ODD of the opposite phase. When the magnetic head is at the position a (cylinder 2), as shown in FIG. 4, the phase difference between the reference phase and the phase of the read pulse of the servo pattern becomes 1/2 of one cycle. It is therefore possible to detect in which one of the cylinders 0 to 3 the magnetic head is positioned by detecting this phase difference.
According to this phase system, after the peaks of the waveform recorded on the servo surface are detected, the phase comparison between the peak pulse and the reference clock is performed a plurality of times and the average phase difference is obtained to be a position signal. As the phase comparison should be executed a plurality of times, signals will not be obtained consecutively. Because of the averaging, however, the detection of the head position is not easily affected by noise. Further, if a level change on the surface of the magnetic disk is on such a degree as not to cause a variation in peak detection, accurate position detection is possible. Since a digital positioning controller has only to obtain position information sample by sample, consecutive information is not necessary so that the above phase type servo pattern is suitable for the digital positioning controller.
According to the prior art, however, a clock generator of a fixed phase type, such as a crystal oscillator, is used to generate a reference clock. The number of rotations of a magnetic disk varies slightly. In using a reference clock of a fixed phase, therefore, when the rotation of a magnetic disk varies, the phase difference between the reference clock and the servo pattern cannot accurately be detected. This will reduce the precision of the position detection. Further, the oscillation frequency of the crystal oscillator varies with temperature. Because the phase of the reference clock varies with the ambient temperature, the phase difference between the reference clock and the servo pattern cannot accurately be detected. This will also reduce the precision of the position detection.
According to the prior art, after detection of the servo pattern, the phase difference and average phase difference are determined to detect the head position by an exclusive processor. When the seek speed of the magnetic head becomes faster, therefore, the position detecting process by the processor will not match the speed. It is therefore difficult to accomplish a faster seek. If a processor with a fast operation speed is used to accomplish a faster seek, the apparatus inevitably becomes expensive.
In addition, according to the prior art, the phase difference changes from 0 to 1 cycle (four clocks) for four cylinders. As the phase of this reference clock is fixed, the phase difference changes by an amount corresponding to four cylinders in accordance with the head position around the center cylinder (cylinder 2 in FIGS. 2 to 4). Accordingly, while there is a consecutive variation width for four clocks around this center cylinder, the variation width is small for the other cylinders (e.g., cylinders 0, 1 and 3). It is therefore difficult to detect the head position in those other cylinders under coarse control, making on-track control difficult.